The nature of non-volatile, vibration-free, small size and low power consumption has made the Flash memory an excellent component to be utilized in various Flash storage devices. Flash storage devices are widely used as memory storage for computer and consumer system products such as notebook, desktop computer, set top box, digital camera, mobile phone, PDA and GPS etc. The increasing demand for more storage in these products has driven the need to expand the capacity of the Flash storage devices.
There are two types of Flash storage devices. The first type has a pre-defined mechanical dimension. This type includes: (a) Secure Digital (SD) card, (b) Multi Media Card (MMC), (c) Memory Stick (MS) card, (d) Compact Flash (CF) card, (e) Express Flash card, (f) Serial ATA Flash disk, (g) IDE Flash disk, (h) SCSI Flash disk, etc.
The second type of Flash storage devices has no pre-defined physical dimension, which includes USB Flash disk, Disk On Module (DOM), MP3 player etc. However, corresponding based upon the need for the system compactness, it is generally desirable to make this type of Flash storage device as small in size and as high in capacity as possible.
Space constraints and available Flash memory density are the major obstacles in expanding the capacity of the Flash storage devices. FIG. 1 illustrates top, bottom, short side lateral and long side lateral views of a secure digital (SD) card 10. The SD card 10 is defined with a form factor of 32×24×2.1 mm (length×width×thick). This fixed dimension restricts the number of components populated on a printed circuit board (PCB) 12. For instance, if TSOP type of Flash memory is used, only a Flash memory chip 14 and a Flash controller 16 can be placed in the space constraint. The available Flash memory density further limits the overall SD card capacity. For instance, if the highest Flash memory is 4 Gb, the maximum SD card capacity is then limited to 512 MB.
A Flash memory die is the basic element of Flash memory. A typical Flash memory chip comprises a Flash memory die mounted on a substrate within an enclosure and the electrical signals are bonded out to the metal contacts of the package. FIG. 2 illustrates a Flash memory chip 50 in a thin, small out-line package (TSOP). The popular package types for flash memory chip are TSOP (Thin Small Out-line Package), WSOP (Very Very Thin Small Out-line Package) and BGA (Ball Grid Array) etc. For the purposes of this application, Flash memory will be used to describe both a Flash memory die and a Flash memory chip.
Besides power and ground, a flash memory includes the following electrical signals:
(a) Bidirectional signals: I/O (Input/Output) bus. It is a bidirectional bus. Flash memory uses this bus to input command, address and data, and to output data during read operation. Multiple Flash memories can share this bus with a Flash controller.
(b) Common Input Control Signals: ALE (Address Latch Enable), CLE (Command Latch Enable), RE- (Read Enable), WE- (Write Enable), WP- (Write Protect). Driven by Flash controller for various operations to Flash memory. These signals are shared among multiple Flash memories connected to a single I/O bus.
(c) Exclusive Input Control Signal: CE- (Chip Enable). Driven by Flash memory controller to enable the Flash memory for access. To ensure only one of them is enabled at a time, each Flash memory is connected to a unique CE-.
(d) Output Status Signals: R/B- (Ready/Busy-). Driven by Flash memory when it is busy, not ready to accept command from the Flash controller. It is an open-drain signal that can be shared among multiple Flash memories connecting to a single I/O bus.
The typical functional block diagram of a Flash storage device 80 is shown in FIG. 3. It comprises a Flash controller 82 and at least a Flash memory 84. One end of the Flash controller 82 interfaces to the host while the other end controls the access to Flash memory 84.
In many instances, due to cost and pin count considerations, a Flash controller has a limited number of chip enable signals. This limitation imposes a restriction on capacity expansion.
Furthermore, as the demand for Flash storage devices has increased, a shortage of certain types of Flash memory occurs during the course of a year. Flash types of the most popular density are typically out of supply during the peak seasons.
Accordingly it is desirable to provide ways to expand the capacity of Flash storage devices. The present invention addresses such a need.